1. Field of the Invention
This invention relates to electrical circuit overcurrent protection.
2. Introduction to the Invention
Positive temperature coefficient (PTC) circuit protection devices are well known. The device is placed in series with a load, and under normal operating conditions is in a low temperature, low resistance state. However, if the current through the PTC device increases excessively, and/or the ambient temperature around the PTC device increases excessively, and/or the normal operating current is maintained for more than the normal operating time, then the PTC device will be xe2x80x9ctripped,xe2x80x9d i.e. converted to a high temperature, high resistance state such that the current is reduced substantially. Generally, the PTC device will remain in the tripped state, even if the current and/or temperature return to their normal levels, until the PTC device has been disconnected from the power source and allowed to cool. Particularly useful PTC devices contain a PTC element which is composed of a PTC conductive polymer, i.e. a composition which comprises (1) an organic polymer, and (2) dispersed, or otherwise distributed, in the polymer, a particulate conductive filler, preferably carbon black and/or a conductive inorganic filler, e.g. a ceramic oxide or a metal carbide, nitride, or boride such as titaniun carbide. PTC conductive polymers and devices containing them are described, for example in U.S. Pat. Nos. 4,237,441, 4,238,812, 4,315,237, 4,317,027, 4,426,633, 4,545,926, 4,689,475, 4,724,417, 4,774,024, 4,780,598, 4,800,253, 4,845,838, 4,857,880, 4,859,836, 4,907,340, 4,924,074, 4,935,156, 4,967,176, 5,049,850, 5,089,801 and 5,378,407, and in International Patent Publication Nos. WO 94/01876, WO 95/08176 and WO 95/31816 (corresponding to U.S. patent application Ser. No. 08/710,925 (Zhang et al, filed Sep. 24, 1996) and Ser. No. 08/727,869 (Graves et al, filed Oct. 8, 1996)), the disclosures of which are incorporated herein by reference for all purposes. Ceramic PTC materials are also well known.
Negative temperature coefficient (NTC) circuit protection devices containing ceramic NTC materials are also well known.
U.S. patent application Ser. No. 08/682,067 describes an overcurrent protection system which will give a rapid response to even relatively small overcurrents. In that system, a sensor element and circuit interruption element are placed in series with the load, and the sensor element is functionally linked to the circuit interruption element via a control element, so that, when the current in the circuit exceeds a predetermined amount, the sensor element senses the overcurrent and communicates with the control element. The control element causes the circuit interruption element to change from a relatively conductive normal state to a relatively non-conductive fault state (including a completely open state). In a preferred embodiment, the sensor element comprises a resistive device connected in series with the load, and the control element comprises a PTC device which is thermally linked to the resistive device and is electrically connected to the circuit interruption element. When an over current passes through such a system, the resistive device increases in temperature causing the PTC device to heat up and trip to its high resistance state. The PTC device is linked to the circuit interruption element so that the increased resistance of the PTC device causes the circuit interruption element to switch into its fault state. The PTC device is not placed in series with the load and therefore may operate at current levels much less than the normal circuit current which passes through the load.
This invention relates to devices which combine a resistive heating element and an element which is thermally linked to the heating element, and exhibits anomalous resistance/temperature behavior. The devices are particularly, but not exclusively, useful in the protection systems described in U.S. patent applications Ser. Nos. 08/682,067 and 08/682,172.
In a first aspect, this invention provides an electrical device which comprises:
(1) a first electrode;
(2) a second electrode;
(3) a third electrode;
(4) a first resistor which is connected in series between the first and second electrodes; and
(5) a second resistor which (i) is thermally coupled to the first resistor, (ii) exhibits anomalous resistance/temperature behavior, and (iii) is connected in series between the first and third electrode.
The first resistor must be one whose temperature changes substantially (as a result of a change in the amount of heat generated within it by resistive heating, also known as Joule or I2R heating) when the current passing through it changes from a first value (usually a normal operating value) to a second value (usually an abnormal, relatively high, fault value). As a result of the change in temperature of the first resistor, the second resistor changes in temperature , and, therefore, in resistance, which in turn substantially changes the current which flows between the first and third electrodes.
The devices of the invention are particularly useful when (a) the second current value is a fault current which is substantially higher than the first current value (which is the normal operating current), and thus causes the first resistor to increase in temperature; and (b) the second resistor is a PTC resistor which increases substantially in resistance as a result of the heat generated within the first resistor, and thus reduces the current which flows between the first and third electrodes. The invention will, therefore, be described chiefly by reference to such devices. It is to be understood, however, that the invention also includes devices as defined above which achieve useful results in a different way, for example devices in which (a) the change in the current reduces the amount of heat generated by the first resistor and thus reduces the temperature of the second resistor; and/or (b) the change in the current through the first resistor is a reduction in size of the current; and/or (c) the change in the current through the resistor is a change in frequency which changes the amount of the heat generated within the first resistor, and/or (d) the first resistor is a voltage-dependent resistor, and/or (e) the second resistor is an NTC resistor.
The devices of the invention are particularly useful in circuits in which (a) the first and second electrodes and the first resistor are in series with the load, and (b) the first and third electrodes and the second resistor are in series with a control element which is coupled to a circuit interruption element so that, when there is an overcurrent through the load, the reduction of current through the control element activates the circuit-interruption element to interrupt, or to reduce substantially, the current through the load. For example, the control element can be a relay coil and the circuit-interruption element a set of relay contacts controlled by the relay coil. Alternatively the control and circuit-interruption elements can be combined in a single three terminal solid state device. For further details and specific examples of such circuits, reference should be made to U.S. application Ser. Nos. 08/682,067 and 08/682,172.
The invention also includes electrical asemblies which can be divided into a plurality of devices of the invention and which preferably comprise
(1) a planar sheet of a composition which exhibits PTC behavior, the sheet having a first major surface and an opposite second major surface;
(2) a first planar metal foil member which is secured to the first major surface of the PTC sheet; and
(3) a second planar metal foil member which is secured to the second major surface of the PTC sheet;
at least one of the first and second metal foil members having been etched so that the assembly comprises a plurality of identical assembly sub-portions, each of the assembly sub-portions
(i) becoming a device as described in the first aspect of the invention when the assembly is divided into a plurality of devices, and
(ii) comprising (a) a metal foil member which is secured to a major surface of the PTC sheet and comprises a first electrode, a second electrode and a resistor connected in series between the first and second electrodes, (b) a sub-portion of the PTC sheet, and (c), secured to the opposite face of the sub-portion of the PTC sheet, a third electrode.